Electromotive Force e Resistência em Circuitos Elétricos, Notas de estudo de Eletrônica

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• Chapters 25.4 and 26 to 26.

How current flows

x 1 2 3

A simple circuit

i

EMF Devices

Ideal EMF device– no internal resistance

Real EMF device– some internal resistance

Internal Resistance

 The battery itself can have some resistance to current flow 

Could be terminals

Could be plates or

paste

Could be

combination

PbO 2 PbO 2 PbO 2 Pb Pb Pb H 2

SO 4

Internal Resistance

 We treat the internal resistance as if an external resistor had been added to the circuit just ahead of the positive terminal

Loop Rule

 The algebraic sum of changes in potential encountered in a complete traversal of the circuit must be zero.  (^) AKA Kirchoff’s Loop Rule  (^) Consider Nashville Panama City Dr. Womble Total Elevation Change = 0

From an electrical perspective

EMF Rule

For a move from the negative terminal to the positive terminal then the change in potential is +EMF

For a move from “+” to “-” then the change in potential is -EMF move +EMF-EMF

Putting these ideas into practice i.e changing a circuit into an equation R=65  EMF=5 V

1. Pick a direction for the current.
2. Pick a direction of circuit traversal
3. Sum the potentials as you traverse the circuit i My move

X

From X, +iR+EMF= EMF=-iR 5=-65i i=-0.076 A or -76 mA The negative sign means that we guessed the wrong direction for the current.

Reducing Networks, If You Can, then DO SO!

Always try to reduce the total number of variables by using the equivalent resistance.

For N resistors in series, the equivalent resistance is

Req=R 1 +R 2 +….+RN

How to find a potential difference

 (^) To find the potential difference between any two points

1. Start at one point

2. Traverse the circuit following any path

3. Add algebraically the changes in potential

R 1

R 2

R 3

E

Point A Point B Blue— Va –iR 3 -iR 2 =Vb Va-Vb=i(R 3 +R 2 ) i Red— Va-E+iR 1 =Vb Va-Vb=E-iR 1

Junction Rule

 Sum of all currents entering a junction must equal the sum of all currents leaving the junction. i 1 i 3 i 2 i 1 i 3 i 2 i 1 i 3 i 2 i 1 +i 3 =i 2 i 1 +i 2 =i 3 i 1 +i 2 +i 3 = IN = OUT

Resistors in Parallel

 (^) Connected resistances are said to be in parallel when a potential difference that is applied across their combination results in that same potential difference across each resistance. E R 1 R 2 R 3 Req

E

E

i 1 i 2 i 3 i i i=i 1 +i 2 +i 3                 1 2 3 1 2 3 1 2 3 1 1 1 1 1 1 1 1 R R R R R R R R R V R V R V R V eq eq eq